Non-CO2 effects of aviation

Non-CO2 effects account for two thirds of aviation’s total climate impact, but no mitigation measures have made it so far in the EU’s aviation climate package.

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What are non-CO2 effects of aviation?

On top of CO
2, aircraft engines emit other gases – nitrous oxides (NOx), sulfur dioxide (SO2) and water (H2O) – and particulate matter (soot). When emitted at high altitudes, these emissions affect atmospheric physical and chemical properties. This results in an increase in greenhouse gases, and the potential formation of persistent contrail cirrus. These high, linear clouds trap the Earth’s heat.

The consequence is a net warming effect, which may be up to three times worse than the warming caused by aviation’s CO2 emissions.

Nevertheless, these effects are also short lived, meaning that acting against them would quickly reduce aviation’s contribution to global warming, scoring important wins in our fight against climate change.

How can we mitigate the non-CO2 impacts of flying?

Persistent contrail formation is mainly the result of soot and other emissions on cold, high humidity atmospheric areas known as Ice Super Saturated Regions (ISSRs). But solutions exist to tackle the problem.

To reduce non-CO2 effects, airplanes can use clean fuels to reduce the amount of pollutants released in the air. Jet fuels with high aromatics and naphthalene concentrations increase soot formation, which in turn leads to persistent contrail cirrus. One way to reduce aromatics and naphthalene in jet fuel is to perform hydrotreating on fossil jet fuels. Reducing aromatics content of fossil jet fuel down to 8-10% can be achieved without significant costs and could lower non-CO2 effects significantly.

Avoiding flying through areas with very cold and humid conditions, known as  Ice Super Saturated Regions (ISSR), is a key axis to reduce non-CO2 effects. Changing flight paths to fly at a lower altitude, or performing small diversions, can avoid contrail formation. For instance, rerouting less than 2% of flights in Japan can reduce the warming effect of contrails by 60%

Airlines must be given information on the regions to avoid so they can plan their flights routes accordingly to minimize persistent contrail cirrus formation. 

What is being done? 

The first piece of evidence highlighting the importance of aviation’s non-CO
2 effects came from the UN’s Intergovernmental Panel on Climate Change (IPCC) back in 1999. Since then, a lot of excellent research has been carried out to better understand the effects of these emissions and how to tackle them.

The European Commission was first tasked with addressing the non-CO2 emissions of flying in 2008, and commissioned a landmark report  to the European Union Aviation Safety Agency (EASA). The report, published in 2020, analyzed the latest available science, quantified non-CO2 climate impact of aviation as twice that of CO2, and proposed some mitigation measures.

However, when reading the EU’s 2030 climate package, known as “Fit for 55”, it’s easy to see that there is barely any mention of non-CO2 effects or the proposed policy measures to address them. 

Failing to address non-CO2 effects in Fit for 55 would be missing a precious opportunity, and the EU cannot afford wasting any more time in tackling the biggest part of aviation climate impact. Policy solutions do exist and can be implemented immediately in the proposal, including in the EUʼs future sustainable fuel mandate (ReFuelEU Aviation) and its updated carbon market rules (EU ETS). As of December 2022, airlines have to disclose their non-CO2 effects, although there are no laws aimed at reducing them.